1. Field of the Invention
The present invention relates to an impact-absorbing structure of a door trim for use in a vehicle such as an automobile. In particular, the present invention relates to an impact-absorbing structure which is adapted to absorb impact through the plastic deformation of ribs which are provided on a door trim when the door member is forced into contact with the ribs by application of impact load to the door member due to a side collision of the vehicle.
2. Description of the Related Art
In a vehicle, in particular, an automobile, numerous structures have been devised in order to protect occupants against a side collision of the vehicle. In general, these structures employ an impact-absorbing member which is located within each door, but is structurally independent of the door. The impact-absorbing member is made of polyurethane foam, polystyrene foam, or the like. Accordingly, the impact-absorbing member is subject to plastic deformation due to the side collision, thereby absorbing impact from the exterior of the vehicle (see Japanese Utility Model Application Laid-Open No. 58-39359.)
However, a drawback is that the above structure is costly because the impact-absorbing member and the door separately form the structure, which increases the number of components. In addition, the many assembly processes involved in the manufacturing of the impact-absorbing member also lead to a rise in the cost of the structure.
A countermeasure against this drawback has been directed to ribs which are positioned on a door trim in order to accomplish an original object of the ribs, i.e., to increase the rigidity of the door trim. Accordingly, the ribs have been designed to be utilized as impact-absorbing members. FIG. 9 through FIG. 11 illustrate cross-sectional views of a conventional door trim in which ribs are used as absorbent members.
The above-mentioned ribs will now be described with reference to FIG. 9 through FIG. 11. In order to utilize the door trim as an impact-absorbing member, lengths of ribs 3 must be large enough to allow the plastic deformation of the ribs 3, as illustrated in FIG. 9. Impact load, which acts on a door member 1, is first transmitted therefrom to a door trim 2, and is then conducted to an occupant via the door trim 2. At the same time, as shown in FIG. 10, the door member 1 is thereby moved toward an interior of a vehicle, and is brought Into contact with the ribs 3 which are mounted to the door trim 2. Impact load is thereby imparted to the door trim 2, and gradually increases. The impact load shortly reaches an initial load where plastic deformation occurs in the ribs 3. This deformation permits the ribs 3 to absorb the impact load that is applied from the exterior of the vehicle. As a result, impact load imparted to the occupant is reduced. The larger the increase in impact load, the greater the amount of the deformation of the ribs 3. As shown in FIG. 11, when a final load is reached over time, the ribs 3 are broken and can no longer absorb the impact.
In order to absorb impact successfully, the greatest value of impact load applied to the occupant must be lowered. However, such a maximum value is variable because a load caused by a collision involves various factors of disturbance. When the rigidity of the ribs 3 is raised to increase the impact-absorbing ability thereof, insufficient or little plastic deformation may occur in the ribs 3 when the maximum value of impact load is small. This case results in insufficient absorbing ability of the ribs 3. In the inverse case, a lower degree of rigidity causes the plastic deformation of the ribs 3 when the maximum value of impact load is smaller. As a result, the impact load can be absorbed. However, the ribs 3 are easily broken when the maximum value of impact load is greater. Further, since the ribs 3 have a smaller impact-absorbing ability in this case, it is difficult to adjust the ribs 3 in order to lower the maximum value of impact load that is applied to the occupant.